We are using the microcirculation of the rat iris as a model, and examining the role played by the vascular adrenotropic beta receptor system in regulating the permeability and/or reactivity of small blood vessels in aging. Vascular smooth muscle contains adrenotropic beta receptors. Stimulation of these receptors causes a dilation of the vessel wall. Furthermore, our experiments show, 1) that stimulation of these receptors increases the permeability of venules, and 2) that there appears to be a decrease in the activity of the beta receptor system in aging. Dose response curves show that the beta agonist isoproterenol causes an increase in the permeability of rat iridial blood vessels. This increase in permeability is greatest in animals less than five weeks old. In both young and older rats, the increase in permeability induced by isoproterenol is prevented by pretreatment with the beta block timolol maleate. We are currently examining the effect of exogenous cAMP on the permeability of iridial blood vessels of young and older animals. However, most of our effort will center around an electron microscopic examination of the effects of beta receptor stimulation upon the ultrastructure of the vascular and nonvascular components of the irises of young and older animals.